Evolution of the Eye

ONE of the classic examples of quote-mining so beloved of creationists is to pluck one of Darwin’s provocative statements out of context — the one where, in the first edition of Origin of Species, Chapter 6, he says:

To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.

Creationists use that sentence to claim that Darwin didn’t believe his own theory. However, either through ignorance or dishonesty, the creationists ignore the fact that Darwin then — in the very next sentence! — presents his explanation:

Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.

Following that, Darwin provides pages of details describing earlier versions of eyes as evidence of their gradual evolution.

Research on the evolution of vision has never stopped. We found an interesting press release at the Public Affairs section of the website of the University of California, Santa Barbara: Scientists at UCSB Discover 600 Million-Year-Old Origins of Vision. Here are some excerpts, with bold added by us:

By studying the hydra, a member of an ancient group of sea creatures that is still flourishing, scientists at UC Santa Barbara have made a discovery in understanding the origins of human vision. The finding is published in this week’s issue of the Proceedings of the Royal Society B, a British journal of biology.

They don’t give a link to the published paper, but we found it for you. Here’s the abstract: The evolution of phototransduction from an ancestral cyclic nucleotide gated pathway.

Let’s continue with the UCSB press release:

Hydra are simple animals that, along with jellyfish, belong to the phylum cnidaria. Cnidarians first emerged 600 million years ago.

Here’s the Wikipedia entry for the hydra. The press release continues:

“We determined which genetic ‘gateway,’ or ion channel, in the hydra is involved in light sensitivity,” said senior author Todd H. Oakley, assistant professor in UCSB’s Department of Ecology, Evolution and Marine Biology. “This is the same gateway that is used in human vision.

[Creationist reaction: "I ain't no kin to no hydra!"]

On with the press release:

Oakley explained that there are many genes involved in vision, and that there is an ion channel gene responsible for starting the neural impulse of vision. This gene controls the entrance and exit of ions; i.e., it acts as a gateway.

The gene, called opsin, is present in vision among vertebrate animals, and is responsible for a different way of seeing than that of animals like flies. The vision of insects emerged later than the visual machinery found in hydra and vertebrate animals.

One last excerpt:

“This work picks up on earlier studies of the hydra in my lab, and continues to challenge the misunderstanding that evolution represents a ladder-like march of progress, with humans at the pinnacle,” said Oakley. “Instead, it illustrates how all organisms –– humans included –– are a complex mix of ancient and new characteristics.”

So there you are. We hope the creationists may find some comfort in knowing that although our vision would appear to descend from the humble hydra, at least we didn’t inherit it from the insects.

Copyright © 2010. The Sensuous Curmudgeon. All rights reserved.

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5 responses to “Evolution of the Eye

  1. Some photosynthetic bacteria behave as a photoreceptor, IOW a simple eye. They are phototactic and these go back at least 2-3 billion years.

    Take that, Cretard!

  2. retiredsciguy

    “… although our vision would appear to descend from the humble hydra, at least we didn’t inherit it from the insects.”

    Zzpeak for yourzelf, Zenzuouz Curmudgeon!

    Actually, the Cnidarians may have emerged way more than 600 million years ago; we just don’t have a good idea because jellyfish didn’t fossilize very well, and unaltered, unmetamorphosed rocks from that far back are rare.

    Thanks for writing such an interesting article.

  3. retiredsciguy says: “Zzpeak for yourzelf, Zenzuouz Curmudgeon!”

    Denial is unbecoming. You should try to get in touch with your inner hydra.

  4. Visual opsins are structurally related to bacterial rhodopsin (although not by amino acid sequence) that probably has an evolutionary history several billion years old. This could arise as a result of amino acid changes over such a large amount of time that the bacterial and animal proteins have lost their sequence similarity, or this could be a great example of convergent evolution where a similar structural solution has arisen independently in bacteria and animals.

  5. I still have scars in my memory from having to participate in science fairs in 8th and 9th grades. At least in 9th grade, I attempted to do the project by the book, trying to set up a proper experiment. I tried to see what would happen if I fed measured doses of vitamins to one population of hydra while not feeding any doses to a control group. Result: both populations died simultaneously when it was too late to get more.

    I think I was getting too fancy with the vitamins. I would have had a better experiment by just cutting the little guys up. (Seriously. They regenerate in the most amusing ways.)